Chlorine dioxide is presently to a large extent used as a bleaching agent for production of fully bleached chemical pulps. In the 1940s it was discovered that chlorine dioxide had excellent ability to remove lighting from cellulosic fibres without noticeably effecting the cellulosic fibre itself. During this period chlorine and sodium hypochlorite were used as main bleaching and delignification agents. The drawbacks with these chemicals were that during the bleaching and delignification step they also attacked and ruptured the cellulose molecule resulting in a weak fibre and ultimately weak paper products.
A combination of chlorine and chlorine dioxide later turned out to give optimum pulp properties regarding strength, brightness and cleanliness.
Within the bleaching technology the following expressions are used: C=chlorine (Cl.sub.2), D=chlorine dioxide (ClO.sub.2), E=extraction with sodium hydroxide (NaOH), O=oxygen (O.sub.2).
Heretofore, chlorine dioxide was usually used at the end of the bleaching sequence. A typical bleaching sequence would be: C E D E D. During the 1960s and 1970s about 5-15% of the chlorine in the first bleaching stage started to be substituted by chlorine dioxide calculated as available chlorine.
The relationship between chlorine dioxide and chlorine within the bleaching field is expressed as follows: 1 kg of chlorine=1 kg of available chlorine. 1 kg of chlorine dioxide=2.63 kg as available chlorine.
The primary reason for adding a small amount of chlorine dioxide in the first bleaching stage was that the fibre strength could be maintained.
During the 1970s, but particularly during the 1980s, the use of chlorinating bleaching agents has more and more been questioned. The primary reason is that when chlorine gas is used as a bleaching agent it produces large amounts of chlorinated organic compounds.
Major experimental work has been done and will continue in order to decrease and perhaps eliminate the use of chlorine gas for bleaching of chemical pulps. Partially for this reason, oxygen delignification has been developed.
Chlorine dioxide also produces chlorinated organic compounds when used as a bleaching agent, but are only a fraction compared to when chlorine gas is used. These compounds are also considered less toxic. For this reason chlorine dioxide is now a candidate for total substitution of chlorine in the first bleaching stage.
As chlorine dioxide is a unique delignification and bleaching agent for chemical pulps with bleaching sequences like O D (E+O) D E D, both chemicals offer maximum utilization with these stages. When going from chlorine to chlorine dioxide bleaching the pulp mill will often have to increase its installed chlorine dioxide capacity substantially. This could lead to a large investment if the old reactor system has to be replaced.
In the beginning chlorine dioxide was produced by reduction of sodium chlorate with sulphur dioxide in sulphuric acid medium according to the so called Holst-procedure. This is a batch process where sodium chlorate solution and sulphuric acid is added to a reactor. A mixture of sulphur dioxide in air was then blown through spargers in the bottom of a reactor. The chlorine dioxide formed was removed from the reaction medium by the sparged air stream and then absorbed in water in a specially designed tower. Later the so called Mathieson Process was developed. Here the reaction components, sodium chlorate, sulphuric acid and sulphur dioxide in air were continuously added to the reactor as shown in FIG. 1. The liquor content from the primary reactor passes through a secondary and sometimes a tertiary reactor where the concentration of sodium chlorate is depleted as much as possible in order to minimize losses. The liquor coming out of the tertiary reactor is called spent acid.
According to other procedures, methanol (the Solvay Process), and chloride (the R-2 Process, R-3 Process, SVP Process, Lurgi Process etc) are being used as reducing agents instead of sulphur dioxide.
When producing chlorine dioxide according to the Mathieson procedure the following reactions are considered to take place ##STR1##
The above reactions are only used as a tentative description of the process. We do not claim that they are a full description of reality. Particularly reaction 2 can be more complicated than what is described here. This reaction is however anticipated to be the one proceeding with the slowest reaction rate and is therefore the rate limiting one. As can be seen from reaction 2 presence of chloride ions are necessary for chlorine dioxide to be formed. Simultaneously chlorine gas is being produced which according to reaction 3 can react with the reducing agent thereby returning to the chloride state.
The reducing agent for the reaction in the reactor can also be methanol according to the Solvay procedure.
In a Mathieson reactor the yield of chlorine dioxide based on sodium chlorate added often is in the range of 82-88%. The yield loss is primarily depending on that chlorate is reduced to chloride. Thereby the reactor is continuously fed with chloride ions to sustain the chlorine dioxide generation. Some chlorine is however also accompanying chlorine dioxide in the gaseous phase to the absorption tower.
It is known that the yield of chlorine dioxide based on sodium chlorate added can be somewhat increased by extra addition of chloride ions to the reactor.
As earlier mentioned it is often a desire both for quality and environmental reasons that the chlorine dioxide produced is free of chlorine. This can be achieved by extra addition of sulphur dioxide which however automatically leads to a greater yield loss of chlorine dioxide with more expensive production costs as a consequence. In order to improve the yield it is known that a scrubber between the chlorine dioxide generator and the absorption tower can be used as shown in FIG. 2. Thereby the solution of sodium chlorate is added to the upper part of the scrubber. This technique, however, has led to an increased number of so called puffs (low velocity detonations of chlorine dioxide) and what is more serious, fires. Attempt to circumvent these problems with extra additions of water to the scrubber leads to dilution of the reactor solution. This demands added volumes of sulphuric acid which increases production costs.
Below are described two known methods of producing chlorine dioxide gas which will provide a better understanding of the invention later described .